The present invention relates to an implant providing osseous anchorage in a vertebra, for example to anchor an osteosynthesis device. This implant has a polyaxial head allowing to receive, in a plurality of angular positions, a bar linking several implants. The present invention also relates to a method for installing the implant, in particular to perform a reduction of a spondylolisthesis (i.e., a jutting out of a vertebra with respect to the adjacent vertebrae).
It is known, in the prior art, to fix a plurality of implants, each to a vertebra, and to connect them together with a bar or a plate, fixed to each implant, along the vertebral column, so as to maintain or correct the rachis. These implants are generally fixed to the vertebrae, either via screwing of a threaded part of the implant in a vertebra, or by fixing of a hook of the implant onto a vertebra. The implants known to the prior art are equipped with a fixation channel, at the head, allowing to fix the bar or plate into a duct. Sometimes, a lateral aperture allows introducing the bar via the side of the duct.
The international patent application WO 03/049629 shows an implant with polyaxial head allowing to orient the bar in different positions before fixing it, these positions being restricted by a determined clearance of the bar around the axis of the implant and/or around the axis of the duct, thanks to a ball and socket connection. This polyaxial system allows to simplify the positioning of the bar and to reduce the constraints exercised by the implant on the bar, but offers a limited number of levels of movement (i.e., a limited freedom of motion of the various elements of the implant with respect to each other).
On the other hand, the implants known in the prior art generally allow to completely reduce a spondylolisthesis of the rachis but do not allow to partially reduce it, a spondylolisthesis reduction consisting in a repositioning of the vertebra in the axis of the adjacent vertebrae in the vertebral column. Yet, in some cases, it is necessary to only partially reduce a spondylolisthesis or to control, during implantation, the reduction level of the spondylolisthesis.
The international patent application WO 00/15125 mentions that an implant with polyaxial head allowing to orient the bar in different positions before fixing it. However, the different embodiments of this implant in the prior art have the inconveniences, when the bar is inserted into its duct, of either not offering all the possible levels of movement, or of not allowing to continue to screw the implant into the vertebrae.